The demand for wireless communication systems has grown steadily, and is still growing, and a number of technological advancement steps have been taken during this growth. In order to acquire increased system capacity and user bit rate for wireless systems by employing uncorrelated propagation paths, MIMO (Multiple Input Multiple Output) systems have been considered to constitute a preferred technology for improving the capacity. MIMO employs a number of separate independent signal paths, for example by means of several transmitting and receiving antennas. Each path communicates a communication stream.
In order for a MIMO system to function efficiently, uncorrelated, or at least essentially uncorrelated, transmitted signals are required. The meaning of the term “uncorrelated signals” in this context is that the radiation patterns are essentially orthogonal. This is made possible for one antenna if that antenna is made for receiving and transmitting in at least two orthogonal polarizations, i.e. polarization diversity. Antennas for MIMO systems may also utilize spatial separation, i.e. physical separation, in order to achieve low correlation between the received signals at the antenna ports. It is also possible to combine spatial separation with polarization diversity, i.e. having at least two antennas separated by 5-10 wavelengths (calculated from the centre frequency of the frequency band for which the antennas are designed), which antennas also are arranged for transmission and reception of signals having orthogonal polarizations.
A base station in a MIMO system may thus be arranged with a number of antennas, separated by 5-10 wavelengths, each one of the base station antennas either being designed for one polarization or two orthogonal polarizations.
However, in an urban environment, where a user equipment (for example a mobile phone or a portable computer) is arranged for communicating with such a base station, a number of objects such as large buildings may block the signal path between the base station and the user equipment, causing a number of reflections. Therefore, the signal to noise ratio (SNR) becomes deteriorated, and thus the signals transmitted between the base station and the user equipment become more and more noisy the more buildings that are in the way.
In WO 2005/064872 a repeater system is described, where a base station having multiple antennas communicates via relay stations. The relay stations relay signals to the user equipment via a number of separate channels, where the channelization for example is in the frequency or time domain. This solution presents a problem, since, if the frequency domain is used, there is a shortage of frequencies to be used due to spectral shortage. If the time domain is used, the system becomes slow.